This will be open to much in way of interpretation and opinion. I would say that what struck me the most in Darwin's work is how he frames evolutionary theory as something to happen over such a long period of time. Prior to Darwin, biologists never actively sought links between modern times and billions of years prior. Darwin helped to bring to light the change of organisms over a wide frame of reference. This and the idea of adaptation, that organisms adapt and appropriate changes to the environment in their own makeup, helped to bring to light a new level of scientific inquiry and thought process that helped to reconfigure how individuals see themselves in the world.

This book documents a great mind's thought processes when he came up with one of the most revolutionary ideas in science. Evolution by Natural Selection came up against the beliefs of most of the world about biology and the creation myth, and is ranked amongst the likes of Copernicus' Model of the Solar System, Galileo's Determination of the Shape of the Earth, and Newton's Law of Gravity.

Darwin was also renowned for being a great rhetoric and he was himself a great writer. The Origin of Species is a compelling read that provides a broken down analysis of all the evidence that Darwin gathered on his trip on the HMS Beagle to the Galapagos Islands. The multivarious adaptations of the animals that Darwin found there are documented faithfully and organized according to their implications for evolution.

Perhaps the greatest idea that Darwin introduced was the fact that natural selection, coupled with other natural factors, contribute to the evolution of a species. And that species do undergo evolution, and that only traits that are heritable contribute to evolution.